The present invention relates to heat exchangers. More specifically, the invention relates to a method of bonding together members of a composite heat exchanger.
An aircraft environmental control system typically includes an air cycle machine and a heat exchanger upstream the air cycle machine. The heat exchanger is used to transfer heat from bleed air to a coolant (e.g., ambient air).
Heat exchangers for aircraft environmental control systems should be lightweight and strong. During operation, the heat exchanger is subjected to large thermal stress and high pressure. The heat exchanger should have sufficient strength to withstand the stress and pressure. Moreover, the walls of the heat exchanger should be made as thin as possible in order to reduce weight. Reducing the weight of the heat exchanger reduces fuel consumption of the aircraft. Reducing fuel consumption, in turn, reduces aircraft operating costs.
The assignee of the present invention has been making aircraft heat exchangers out of carbon/carbon composites. Carbon/carbon composites offer high strength and light weight.
A method of manufacturing a carbon/carbon composite plate-fin heat exchanger core is disclosed in Anderson et al. U.S. Ser. No. 08/601,754 filed Apr. 12, 1996, entitled "CARBON/CARBON HEAT EXCHANGER MANUFACTURING METHOD" and assigned to the assignee of the present invention. A first plurality of woven sheets of low modulus Pitch carbon fibers are impregnated with resin and shaped into fins. A second plurality of sheets of carbon fibers are impregnated with resin and shaped into plates. The resin-impregnated fins are stacked between the resin-impregnated plates to form a stacked assembly, the stacked assembly is then cured, pyrolized, densified and annealed to form a fully graphitized plate-fin heat exchanger core.
Joints between plate and fin tends to be the weakest part of the composite heat exchanger core. It is most likely to separate, especially if subjected to large thermal stress and high pressure. The plate-fin joints also tend to have the lowest thermal conductivity and, therefore, do not maximize the transfer of heat from the bleed air to the coolant.
It would be desirable to increase joint strength. It would also be desirable to increase joint thermal conductivity.